1. Field of the Invention
The present invention relates to a positive planographic printing plate precursor having improved solubility in an aqueous alkaline solution after exposure to infrared light, and more specifically relates to a positive planographic printing plate precursor capable of directly making a plate by scanning with an infrared laser light based on digital signals from computers and the like, that is, capable of direct plate-making.
2. Description of the Related Art
Lasers have developed remarkably in recent years, particularly, there has been progress in higher-output smaller-sized solid-state lasers and semiconductor lasers having a light emission region in the near-infrared to infrared regions. Accordingly, such lasers are extremely useful as exposure light sources for direct plate-making using digital data from computers and the like.
The materials for making positive planographic printing plates for use with infrared lasers employ an infrared laser having a light emission region in the infrared region as an exposure light source, and include a binder resin soluble in an aqueous alkaline solution, a light-absorbing heat-generating IR dye and the like as essential components. The IR dye and the like in the materials function as a dissolution inhibitor for substantially lowering the solubility of the binder resin in the unexposed areas (image areas) due to the interaction of the IR dye and the like with the binder resin. On the other hand, when these materials are exposed to infrared laser light, in the exposed areas (non-image areas), the IR dye and the like absorbs the light and generate heat which weakens the interaction between the IR dye and the like and the binder resin. Accordingly, during development, the exposed areas (non-image areas) are dissolved in an alkaline developer and a planographic printing plate is formed.
However, these materials for positive planographic printing plates for use with infrared lasers have problems since the binder resin thereof must have higher solubility in a solvent such as an alkaline developer, as compared with the materials for a positive planographic printing plate material for use with ultraviolet light exposure. These problems are poorer endurance of the plate against a plate cleaner which is used when ink adherence deteriorates during printing, which results in dissolution of photosensitive components when the surface of the plate is wiped by the plate cleaner.
For improving endurance of the plate against the plate cleaner, a photosensitive composition using a polymer containing a structural unit having a specified functional group capable of forming a hydrogen bond to a photosensitive layer has been proposed (for example, refer to the specification of U.S. Pat. No. 6,475,692), for which improvement in endurance against the plate cleaner has been reported.
However, the positive planographic printing plate precursor for use with infrared lasers described above has a problem in that the strength of the photosensitive layer itself is low and printing endurance is poor. Further, although the polymer described in the patent literature has a function of widening the difference in alkaline solubility (discrimination) between the exposed and unexposed areas, the effect is insufficient and a technique is desired for further improving the stability of development (image development latitude) in spite of variations in usage conditions.
Further, although a technique of adding a specified surfactant to a positive recording layer has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 2002-122982) with an aim of improving the discrimination, the surface of the recording layer is then rendered hydrophilic due to the inclusion of a hydrophilic surfactant in the recording layer, thus resulting in concerns about a lowering of resistance to alkaline developer in the image area.